For electrical charges racing through an atom-thick sheet of
graphene, occasional hills and valleys are no big deal, but the
potholes—single-atom defects in the
crystal—they’re killers. That’s one of
the conclusions reached by researchers from the National
Institute of Standards and Technology (NIST)
and the Georgia Institute of Technology who created detailed maps of
electron interference patterns in graphene to understand how defects in
the two-dimensional carbon crystal affect charge flow through the
material. The results, appearing in the July 13 issue of Science*, have
implications for the design of graphene-based nanoelectronics.
A single layer of carbon atoms tightly arranged in a honeycomb
pattern, graphene was long thought to be an interesting theoretical
concept that was impossible in practice—it would be too
unstable, and crumple into some other configuration. The discovery, in
2004, that graphene actually could exist touched off a rush of
experimentation to explore its properties. Graphene has been described
as a carbon nanotube unrolled, and shares some of the unique properties
of nanotubes. In particular, it’s a so-called ballistic
conductor, meaning that electrons flow through it at high speed, like
photons through a vacuum, with virtually no collisions with the atoms
in the crystal. This makes it a potentially outstanding conductor for
wires and other elements in nanoscale electronics.
Defects or irregularities in the graphene crystal, however,
can cause the electrons to bounce back or scatter, the equivalent of
electrical resistance, so one key issue is just what sort of defects
cause scattering, and how much. To answer this, the NIST-Georgia Tech
team grew layers of graphene on wafers of silicon carbide crystals and
mapped the sheets with a custom-built scanning tunneling microscope
(STM) in the NIST Center for Nanoscale Science and Technology that can
measure both physical surface features and the interference patterns
caused by electrons scattering in the crystal. (Graphene on silicon
carbide is a leading candidate for graphene-based nanoelectronics.)
The results are counter-intuitive. Irregularities in the
underlying silicon carbide cause bumps and dips in the graphene sheet
that lies over it rather like a blanket on a lumpy bed, but these
relatively large bumps have only a minor effect on the
electron’s passage. In contrast, missing carbon atoms in the
crystal lattice cause strong scattering, the interference patterns
Comparison of an STM topographic image of a section of
graphene sheet (top left) with spectroscopy images of electron
interference at three different energies shows strong interference
patterns generated by rippling around them like waves hitting the piles
of a pier. From a detailed analysis of these interference patterns, the
team verified that electrons in the graphene sheet behave like photons,
even at the nanometer scale.